1. Field of the Invention
This invention relates to a shading-compensation method and device for effectively eliminating shading-distortion resulting from nonlinear characteristics of optical systems and image pickup elements in image processing devices of various types, and more particularly to a shading-compensation method and device capable of carrying out image reading operations at high speed by efficiently reading out comparative values for shading-compensation which are beforehand stored in memory means.
2. Description of the Prior Art
In performing image processing with image scanners, facsimiles, copying machines or the like or effecting pattern recognition, an objective image given to be read is generally subjected to continuous scanning by one sub-scanning lines by use of a line sensor such as a CCD image sensor. For illuminating a reading portion of the objective image in order to read optically image information with the line sensor, a light source such as a fluorescent lamp and light-emitting diode array is generally used together with an optical system including lenses. However, every lens as used in such an optical system inevitably gives rise to shading-distortion because the intensity of the light which is reflected on the objective image given to be read and passes through the lens is decreased toward the peripheral portion of the lens. In order for eliminating the shading-distortion, there has been conventionally used a shading compensating plate which has light-transmission factor gradually increasing from the center toward the peripheral portion thereof, as proposed in Japanese Patent Publication Gazelle SHO 53(1978)-14329 and Japanese Utility Model Public Disclosures SHO 56(1981)-93061(A) and SHO 56(1981)-137563(A).
Also as disclosed in Japanese Patent Public Disclosure SHO 61(1986)-48982, the intensity of light emitted from a light source array might be made uniform by arraying light-emitting diodes in a line so as to increase the distance between the adjacent diodes going from the center portion toward the end portions of the diode array.
Although, in any of the prior art as mentioned above, the shading-compensation characteristic is invariable, the nonlinearity in light distribution of the light source is not always maintained constant. For instance, the distribution and intensity of the light emitted from the light source is subjected to a secular change. The change of intensity is apt to occur particularly in a rod-shaped illuminating means such as a fluorescent lamp. To be more exact, reflectance of a picture element at a specific column in each sub-scanning line on an undefiled surface given as a standard image is subtly different from those at the same column in the other lines.
Furthermore, the intensity of light reflected on the same pixel in the same sub-scanning line subtly varies because of indefinite optical factors such as irregularity of the optical system each time the same standard image surface is optically scanned. This is why the aforesaid shading-distortion is brought about. Although such shading-distortion must be eliminated to perform image processing with good reproductivity of image, however, the conventional shading-compensation methods as noted above could not effect the thorough shading-compensation, thereby to incur deterioration of the quality of a resultantly reproduced image obtained finally.
One example of the conventional methods for effecting compensation of the aforementioned shading-distortion will be described with reference to a block circuit diagram shown in FIG. 1. First, an objective image 1 given to be read is optically scanned with scanning means 2 to produce an image light b reflected on the objective image 1. The image light b is converted to an electric signal by a line sensor 3 such as a CCD image sensor and quantized by an analog-to-digital converter (A/D) 4, consequently to obtain an image data signal Si. In a processing unit 5, the image data signal Si thus obtained is compared with values VW, VB of reference data for white and black picture elements, which values are beforehand stored in memory 6, so that the image data signal Si is processed to be subjected to the shading-compensation. As a result, a compensated signal So is obtained and outputted to an external image processing device 9 via a buffer 7 and an interface circuit (I/F) 8.
In the aforementioned prior art method for effecting the shading-compensation, after transferring to the processing unit the image data signal Si which is produced as a result of optically reading the n'th picture element in a specific sub-scanning line virtually defined on the objective image, the reference values VWn, VBn for the n'th picture element are respectively read out from white-reference memory 6w and black-reference memory 6b and sent to the processing unit 5. Thereupon, the shading-compensation processing is effected. That is to say, as shown in the timing chart of FIG. 2, the image scanning for the n'th picture element and the data reading-out of the reference values of two sorts from the memory 5 are simultaneously carried out in the period t1-t2 and the arithmetic processing for comparing the read-out data signal with the respective reference values is carried out for the shading-compensation in the period t2-t3. Upon completion of the shading-compensation processing for n'th picture element, the n+1'th picture element is consecutively subjected to shading-compensation processing.
Thus, the conventional method has entailed a disadvantage in that the time lag of t1-t2 (transition time) occurs between the reading processes for the n'th and the n+1'th picture elements. Because the time lag required for the transition of the reading processes to the subsequent picture element occurs repeatedly every picture element until finishing the scanning of the whole image surface, the image reading processes consumes much time and remarkably reduces the processing rate.